That does not appear to be using any kind of specific flat-shading approach, at all. The shading is smooth, though the projected shadows are hard-edged, and the objects are not rounded.
The specific lighting effect appears to be a combination of ambient occlusion (probably SSAO, "screen space ambient occlusion") which is what gives it that soft shadowing in crevices and regular ol' hard-edged shadow mapping and a plain lighting model (probably just Phong lighting).
An orthographic projection is being used. The only light appears to be a directional light which combined with all the objects being squared and flat is why each face has a mostly uniform shade (not any kind of special flat-shading technique).